1. Field of the Invention
The present invention relates to improvements in a seismic isolation apparatus for providing seismic isolation for and supporting a building structure, an exhibit case, various machines and equipment, a floor, or the like.
2. Description of the Related Art
As a seismic isolation system or a vibration control device adapted to absorb seismic energy at the time of occurrence of an earthquake and control the transmission of vibration to an upper portion, ones having various structures have hitherto been developed. The assignee of the present invention has also developed an apparatus adopting a seismic isolation structure which is comprised of roller bearings, as disclosed in, for example, Japanese Patent Application Publication No. 6490/1996. As shown in FIG. 11, this apparatus has a structure in which a seismic isolation mechanism A is formed by interposing a roller 61 with a circular cross section between concave circular arc surfaces 51 and 52 which are formed on opposing surfaces of a pair of upper and lower clamping members 41 and 42, a plurality of these seismic isolation mechanisms A are arranged in an identical plane in a state in which the rolling directions of their rollers 61 are set in the same direction, and seismic isolation mechanisms A each having the same structure are integrally arranged above these seismic isolation mechanisms A in a state in which the rolling direction of their rollers 61 is set in a direction perpendicular to the rolling direction of the rollers 61 of the aforementioned seismic isolation mechanisms A.
According to the seismic isolation mechanism arranged as described above, the transmission of seismic waves to the upper clamping member 41 can be controlled by absorbing horizontal vibrations acting in the lower clamping member 42 by the roller 61 which rolls in contact with the concave upper and lower circular arc surfaces 51 and 52, and the upper clamping member 41 can be restored to its original position when the earthquake is subsided. However, since the concave upper and lower circular arc surfaces 51 and 52 which clamps the roller 61 are formed with curves surfaces having a fixed identical radius of curvature, if the roller 61 has rolled in contact with the opposing circular arc surfaces 51 and 52 of the upper and lower clamping members while a loaded object C on the seismic isolation apparatus is being lifted up in a stationary state in which the roller 61 is at its lowest position, a restoring force F of the loaded object C on the seismic isolation apparatus increases linearly in proportion to a rolling distance X of the roller 61, as shown by solid lines in FIG. 12, and the spring constant (restoring characteristic) of a supporting portion based on the roller 61 becomes fixed. Accordingly, this seismic isolation apparatus is provided with a fixed natural frequency, and if the seismic waves of the earthquake approach the neighborhood of this natural frequency, the phenomenon of noticeable resonance occurs, so that there is a problem in that, contrary to the vibration isolating effect, a vibration exciting effect is exhibited with respect to an object placed on the seismic isolation apparatus.
In addition, the above-described seismic isolation apparatus must be assembled such that, by using the plurality of single seismic isolation mechanisms A in each of which the roller 61 with a circular cross section is interposed between the concave circular arc surfaces 51 and 52 which are formed on opposing surfaces of the pair of upper and lower clamping members 41 and 42, these seismic isolation mechanisms A are arranged at, for example, four sides with fixed intervals provided between them in the longitudinal and transverse directions, an intermediate plate B is disposed on these seismic isolation mechanisms A arranged at the four sides, and the other seismic isolation mechanisms A are arranged on this intermediate plate B in a state in which the rolling direction of their rollers 61 is set in a direction perpendicular to the rolling direction of the rollers 61 of the lower seismic isolation mechanisms A. Therefore, much labor and time are required for the installation operation. Moreover, all the seismic isolation mechanisms A that are arranged at the four sides must be set such that their rollers 61 roll in the same direction, so that considerable time and trouble as well as expertise are required. Furthermore, there are problems in that the overall thickness (height) of the apparatus becomes large, which not only impairs the outer appearance but can impose a restraint on the condition of use, and an impression is given that the loaded object C is placed in an unstable state.
Further, the structure provided is such that the concave circular arc surfaces 51 and 52 are provided on the overall opposing surfaces of the clamping members 41 and 42, while the roller 61 over its substantially entire length is in contact with the circular arc surfaces 51 and 52 in their widthwise direction. Therefore, if the diameter of the roller 61 varies even slightly in its lengthwise direction, or if there are very small projections on its peripheral surface, the roller 61 rattles and can be possibly deflected in the axial direction. For this reason, there arises a need to separately provide a guide groove for allowing the roller 61 to roll in an accurate direction, with the result that there is a problem in that the overall apparatus becomes complex.